Oxidation number is conceived to be the charge associated with a given atom, when it donates or accepts electrons according to predetermined rules...given here for your chemical pleasure....
#"1. The oxidation number of a free element is always 0."#
#"2. The oxidation number of a mono-atomic ion is equal"# #"to the charge of the ion."#
#"3. For a given bond, X-Y, the bond is split to give "X^+# #"and"# #Y^-#, #"where Y is more electronegative than X."#
#"4. The oxidation number of H is +1, but it is -1 in when"# #"combined with less electronegative elements."#
#"5. The oxidation number of O in its"# compounds #"is usually -2, but it is -1 in peroxides."#
#"6. The oxidation number of a Group 1 element"# #"in a compound is +1."#
#"7. The oxidation number of a Group 2 element in"# #"a compound is +2."#
#"8. The oxidation number of a Group 17 element in a binary compound is -1."#
#"9. The sum of the oxidation numbers of all of the atoms"# #"in a neutral compound is 0."#
#"10. The sum of the oxidation numbers in a polyatomic ion"# #"is equal to the charge of the ion."#
And thus for elemental sodium we gots #Na(0)#.. Sodium is a good reductant, and for most of its reaction chemistry we get #Na^+ -=Na(I+)#... In the given scenario, the metal has NEITHER accepted nor donated its single valence electron...
